Astronomers have found the first object, other than the planets Mercury and Venus, whose orbit about the Sun is completely inside the Earth's orbit.

The object, designated 2003 CP20 was detected on 10 February by the Linear automated sky survey telescope. It is a rocky body estimated to be few km in size.

Brian Marsden of the Minor Planet Center at Harvard told BBC News Online that the object "seems to be the first one with an orbit entirely inside that of the earth (apart from Mercury and Venus)".

The object is thought to be the first of many such asteroids that could be detected in the future.

No collision

According to Dr Marsden, 2003 CP20 is the first confirmed asteroid in such an orbit. A previous candidate was found and then lost.

2002 CP20 does not come particularly close to the Earth, because of the 25 deg inclination of its orbit around the Sun.

Researchers think that the object was once a main belt asteroid circling the Sun between the orbits of Mars and Jupiter.

It was then perturbed into a trajectory that took it closer to the Sun where the planet Venus, it is supposed, captured the object into its present orbit.

German astronomer Reiner Stoss is delighted to have been able to image the elusive object.

"I'm happy that this population of objects has finally been found," he told BBC News Online, "especially because I too put a bit of effort in searching for them back in 1998/1999, but I cancelled my search when I noticed how much sky the Linear sky survey was covering."